1. Technical Field of the Invention
The present invention generally relates to analog to digital converters (ADC). More particularly, it relates to photonic analog-to-digital converters using a new pipeline scheme utilizing wavelength division multiplexing and distributed phase modulation. The present invention performs analog-to-digital conversion within the optical system, and thus produces a binary optical output in the form of a digital bit pattern.
2. Description of Related Art
It is often desirable to convert an analog amplitude varying signal to a digital set of values which correspond to various voltages in the analog waveform to generate a corresponding digital signal. Conventional approaches rely on iterative or comparative techniques for determining a digital signal based on an analog waveform voltage. Further, transformation of wideband data signals from the analog-to-digital domain may require sample rates that are generally not available in pure electronic analog-to-digital converters. Using currently available technology, electronic ADCs are limited to about 2 gigasamples/second. Ideally, the sample rate of a suitable ADC should be from 2.5 to 4 times the maximum bandwidth of the analog signal digitized.
Presently, the fastest commercially available ADCs are flash converters, which comprise of a sample and hold circuit and a digitizer circuit. By using multiplexing or interlveaving techniques, the sample rate of such electronic Analog-to-Digital (A-D) systems maybe extended to about two gigasamples/second at about 6-bits of resolution. Some ADCs architectures involved interleaving parallel channels of sampling and comparing circuits, thereby increasing their effective speed by the number of parallel channels.
Recognizing the limitations on bandwidth, sampling rate, and resolution of electronic ADCs, focus of the prior art shifted to optical devices to substantially improve upon these parameters resulting in electronic ADCs evolving in several different architectures. Photonic systems, with their large bandwidths and low-noise operation, may be directly substituted for their electronic counterparts, thus improving the integrated system and thereby extending the overall performance.
Although the concept of photonic ADC is relatively old, most of the current photonic ADCs involve techniques that use mode-locked lasers to provide picosecond and sub-picosecond sampling of the electronic waveform in an electro-optic device, such as an electro-optic modulator (EOM). These systems operate by using photonic sampling as a high-speed, photonic sample-and-hold circuit, or as a pre-sampler for an electronic sample-and-hold circuit, thus enhancing the performance of the final-stage electronic ADC which performs the digitization of the signal.
To further enhance the speed of the entire electronic and photonic system, the photonic system samples the waveform at a much higher speed, and then divides or demultiplexes the output onto several channels that operate at the speed of the electronic ADCs. The electronic output data is then interleaved to provide an effective sampling rate that is higher than the rate that could be achieved with a single electronic ADC. The capability of the photonic system to operate at very high sampling rates arises from the mode-locked laser sources that have extremely precise timing capable of serving as an optical clock. While the prior art teaches improving the speed of the ADCs, it fails to teach or suggest a pipeline or a series scheme for photonic ADCs utilizing wavelength division multiplexing (WDM) and distributed phase modulation.
In U.S. Pat. No. 5,010,346 to Hamilton et al., an electro-optical A-D converter is disclosed which uses a series of separate lasers having different wavelengths as an optical carrier. It will be appreciated that it would be difficult to synchronize the timing and amplitude of these laser beams, and that the resulting jitter between the channels limits the sampling rate and amplitude resolution. Moreover, the number of channels such an electro-optical A-D converter can use appears to be limited to about ten.
In U.S. Pat. No. 4,502,037 to Le Parquier et al. discloses an A-D converter that includes an optical modulator which includes one interferometer channel for each bit of a digital output word is disclosed. The output word corresponds to the magnitude of an analog input signal. The modulator applies a phase shift to each channel used to modulate light from a laser source, and the modulated light is demodulated by an array of photodetectors and comparators to produce a corresponding digital signal.
In U.S. Statutory Invention Registration H353 an optical converter with expanded dynamic range is disclosed. The expanded dynamic range is achieved by dividing the input signal into an optically modulated light pulse signal comprising least significant bits (LSB) and most significant bits (MSB) representations. The LSB and MSB representations are then interleaved to form a final binary representation of the input analog signal. In this system, each of the parallel optical channels are created with electro-optic modulators, and these modulators are driven in parallel by an input analog signal.
Based on the foregoing, it should be appreciated that there has arisen a need for an apparatus and method for converting an analog signal into a digital signal by using a new pipeline scheme utilizing wavelength division multiplexing and distributed phase modulation, and thus removing the complexity of channel timing and synchronization of the time-interleaved photonic ADCs. Furthermore, the digital output of the analog-to-digital converter system may be beneficial since it allows direct optical data transfer through transmission media in telecommunication systems.
Accordingly, the present invention is directed to an innovative solution which provides for the conversion of analog signals to digital signals using a series of electro-optic modulators (EOMs). The present invention adopts wavelength division multiplexing and distributed phase modulation techniques to perform analog-to-digital conversion of an input signal, wherein analog-to-digital conversion is performed within the optical system of the analog-to-digital converter. In the preferred embodiment of the present invention, the EOMs are essentially electro-optic phase modulators (EOPMs) used to change/modify the phase/polarization of an optical signal.
In one aspect, the present invention is directed to a photonic analog-to-digital converter (ADC) system which includes a multiwavelength optical source capable of producing signals of different wavelengths, and at least one polarizer in order to set a polarization state within the photonic analog-to-digital converter system. A plurality of electro-optic modulators are arranged in series wherein each of the electro-optic modulators (EOMs) perform signal processing to produce an optical output having a modified polarization state. A plurality of wavelength filters are arranged in series to probe an optical phase change, wherein each of the wavelength filters extracts an optical signal of a specified wavelength. A plurality of polarization controllers set an optical bias within the ADC system. The ADC system further includes a plurality of analyzers for analyzing polarization states to create an optical transfer function, wherein each analyzer analyzes the polarization state of a respective EOM.
In one exemplary embodiment of the present invention, the wavelength filter is comprised of a wavelength division multiplexer (WDM). In another exemplary embodiment, each analyzer is comprised of a polarization maintaining optical isolator. In yet another exemplary embodiment, each of the wavelength filters is comprised of a beamsplitter to filter optical signals of a specified wavelength. It is to be noted that a single wavelength source, instead of a multiwavelength source, may be used in the embodiment where a beamsplitter is employed to filter optical signals.
In another aspect, the present is directed to a method of converting analog signals to digital signals in a photonic analog-to-digital converter system comprising at least one polarizer, a plurality of electro-optic modulators arranged in series, and a plurality of wavelength filters to probe an optical phase change. First, a plurality optical signals of differing wavelengths are produced using a multiwavelength optical source. These multiwavelength signals are passed through a polarizer in order to set an initial polarization state in the ADC system. The polarized optical signals are passed through an electro-optic modulator which modifies the polarization states of the optical signals, thus resulting in an optical signal with modified polarization states. The modified optical signal is then passed through a wavelength filter, disposed in between a plurality of electro-optic modulators, in order to extract an optical signal of a specified wavelength.
The extracted optical signal is subsequently processed whereby a change in polarization is converted to a change in optical intensity to produce an individual binary optical output representing a most significant bit in the digitized representation of the analog signal. The unextracted optical signals are passed through a second electrooptic modulator to produce a modified optical signal, and the modified optical signal is passed through a second wavelength filter to extract an optical signal of another specified wavelength. The extracted signal is likewise processed to produce an individual binary optical output. The above method continues until signals of all the wavelengths are extracted and processed resulting in binary optical outputs. The combination of all the binary optical outputs produces a digital equivalent value of the analog signal.
In yet another aspect of the invention, the present invention is directed to a photonic analog-to-digital converter (ADC) integrated into a single electro-optic crystal. The photonic ADC system integrated into a single electro-optic crystal includes a at least one polarizer for setting the polaization state within the photonic ADC system, at least one analyzer to perform signal processing to produce a polarization signal having a modified polarization state. Optical signal processing components, such as wavelength filters, etc., are also disposed at regular intervals in the single crystal in order to filter optical signals of specified wavelengths.
The wavelength filters may be used to probe an optical phase change with each of the wavelength filter extracting an optical signal of a specified wavelength. A plurality of polarization controllers are also provided in the single electro-optic crystal to set an optical bias within the ADC system. The photonic ADC system further includes at least one analyzer for analyzing polarization states to create an optical transfer function by analyzing the polarization state of a respective electro-optic modulator.